Large industrial buildings have unique heating requirements. They can be serviced by large central heating facilities or, as frequently is the case, they can be heated by a series of ceiling mounted unit heaters. This is particularly the case with factories and warehouses. Unit heaters eliminate the need for duct work which is expensive and takes up space limiting the utility of the building.
Until now these unit heaters have all been indirect fired units. Indirect fired unit heaters burn fuel in a combustion chamber and the combustion products pass through a plenum and subsequently through a flue to the exterior of the building. The plenum acts as a heat exchanger and heat is drawn off and blown into a room. Unfortunately, these indirect fired units are generally very inefficient. Some are fifty percent efficient or less.
Direct fired units are generally ninety-eight to one-hundred percent efficient. Direct fired units burn fuel in combination with exterior air and the combustion products are discharged directly into the area being heated. Thus, no heat is lost up the flue. Unfortunately, there are no direct fired heaters that are adapted to replace unit heaters. Existing direct fired heaters for factories are large central heaters designed to supply all the heat requirements for the building and requiring duct work.
Since a factory typically incorporates many different unit heaters spaced around the building, when one breaks down it is not economically feasible to replace all the existing unit heaters with one direct fired central heater. Thus, generally with buildings that have indirect fired unit heaters, when one breaks down, they will typically replace it with an indirect fired unit heater.
For various reasons, it is difficult to design a direct fired unit heater. First of all, these must burn outside air. If they burned interior air and discharged the combustion products, the carbon dioxide and carbon monoxide levels would quickly increase until lethal. Further, there are stringent mixing requirements for unit heaters. These must mix at least seventy-five percent interior air with twenty-five percent combustion products. Also, unit heaters must be designed to be supported from the ceiling which adds another difficulty in designing a direct fired unit heater which can be easily serviced. All-in-all, direct fired heaters are generally much more complex than an indirect fired heater.